BY TJ L'HEUREUX, ADRIENNE WASHINGTON, ALBERT SERNA JR., ANISA SHABIR, ISAAC STONE SIMONELLI, THE HOWARD CENTER FOR INVESTIGATIVE JOURNALISM AND ARIZONA CENTER FOR INVESTIGATIVE REPORTING GRAND RAPIDS, Mich. (AP) - Against the background hum of the convention center, Dar Leaf settled into a club chair to explain the sacred mission of America's sheriffs, his bright blue eyes and warm smile belying the intensity of the cause. "The sheriff is supposed to